1. Field of the Invention
This invention relates to an apparatus and a method of performing a baking step in the photolithographic processes on a semiconductor substrate, a substrate of a liquid crystal display, and a substrate of photomask used for forming these substrates when the substrates are formed.
2. Description of the Related Art
The photomask used in semiconductor devices is obtained by a series of steps: forming of a thin film of metal such as Cr by use of the sputtering deposition; applying of a photoresist thereon; prebaking; writing of a pattern with an electron beam writing apparatus; developing of the photoresist; postbaking; performing of a selective-etching of the thin metal film; and removing of the photoresist.
In the conventional photolithography, the resolution or size uniformity of pattern on the surface of the substrate is not required so strictly. In addition, the characteristics of the photoresist used in the photolithograpy do not much depend on the uneven distribution of heat applied to the photoresist in the baking process.
The conventional baking apparatus for baking a photoresist will be more specifically described below, in conjunction with the sectional view shown in FIG. 1. As shown in the drawing, the conventional baking apparatus has only one heating plate, i.e., heating plate 71. A substrate 72 made of material such as quartz and coated with photoresist is located above the heating plate 71 so as to be separated therefrom by a proximity gap of several microns. The photoresist is baked for a predetermined period of time. The proximity gap is provided by a sealing member 73 between the substrate and the heating plate. As can be understood from this structure, the substrate 72 is placed in an open space while the photoresist is being baked in the conventional baking apparatus. The substrate is removed from the heating plate 71 so that the photoresist may be cooled at room temperature.
In recent years, as the semiconductor device is decreased in size, higher resolution and higher size uniformity are required of the pattern on the substrate. Simultaneously, a chemical amplification electron-beam resist which can serve to provide a high-resolution pattern is put into practical use. SAL 601, SAL 603, SAL 605 developed by Shipley Co., are the most popular products of this type. Phenomenons such as crosslinking reaction (in the negative type photoresist) and decomposing reaction (in the positive type photoresist) are known to occur in this type of photoresist. As generally known, these reactions greatly influence the size of the resist pattern during the heating step in the baking process. In order to prevent the influence of these reactions in the chemical amplification electron-beam resist, the baking apparatus capable of attaining the high size uniformity of the pattern is required.
However, in the conventional baking apparatus, the heating plate is directly influenced by the ambient air flows and the heat source. A large difference in temperature occurs in the surface of the heating plate, i.e., between the central portion and the peripheral portion. When a 6 inch mask (a quartz substrate of a size of 152.4.times.152.4.times.6.4 [mm]) is subjected to the baking step at 120.degree. C., for example, the maximum temperature difference in the substrate is no less than 29.0.degree. C. at the transition time the baking temperature is increasing (in this case, when the baking temperature is 63.degree. C.). At the climax time the temperature stably remains at 109.degree. C., the maximum temperature difference is 7.9.degree. C. As is clear from this, the maximum temperature difference in the substrate surface by use of the conventional baking apparatus is considerably large both at the climax and during the temperature transition.
A chemical amplification electron beam resist is generally baked at approximately 100.degree. C. to have its sensitivity amplified. At such a temperature, the sizes of the pattern will greatly change due to the temperature difference in the surface of the substrate. The difference in the pattern size will greatly increase after the steps following the baking step (i.e., the developing of the photoresist, selective etching of the thin metal film, and removing of the photoresist) are performed. The resultant substrate submitted to such a process does not satisfy the manufacturing criteria, of course. This is thus critical problem to be solved urgently.
Some conventional baking process is performed without providing the aforementioned proximity gap between the heating plate and the substrate. In this case, the maximum temperature difference increases more.
Further, since the heating plate of the conventional baking apparatus is exposed to the outside, as shown in the drawing, and dust may stick to the photoresist during the baking process. This structure of the conventional baking apparatus is thus also one of the causes of degradation of the photomask.
As described above, with the conventional baking apparatus, the maximum temperature difference in the substrate surface increases during the baking step, and the pattern size of the photomask (final product) greatly differs from the designed one.